1. Field of the Invention
The present invention relates to semiconductor devices and to methods of manufacturing semiconductor devices.
2. Description of the Related Art
In recent years, there has been extensive production of semiconductor devices in which a thin film transistor (a TFT) is formed over a substrate, such as a glass substrate, which has an insulating surface, and used as a switching element or the like. Over a substrate having an insulating surface, an island-shaped semiconductor layer is formed using a CVD method, a photolithography process, or the like, and the thin film transistor is provided such that part of the island-shaped semiconductor layer is used as a channel forming region of the transistor (for example, see Reference 1: Japanese Published Patent Application No. H08-018055).
Schematic diagrams of a common thin film transistor are shown in FIGS. 24A to 24D. FIG. 24A shows a top view of a thin film transistor. FIG. 24B shows a cross section taken along the broken line joining A1 and B1 in FIG. 24A. FIG. 24C shows a cross section taken along the broken line joining A2 and B2 in FIG. 24A. FIG. 24D shows an enlarged view of an end portion 25 of a semiconductor layer 32 in FIG. 24C. As shown in FIGS. 24B to 24D, in the thin film transistor, an insulating layer 31 which serves as a base film is formed over a substrate 30; the semiconductor layer 32, which includes a channel forming region 32a and impurity regions 32b and 32c which serve as a source region and a drain region, is formed over the insulating layer 31; an insulating layer 33 which serves as a gate insulating layer is formed over the semiconductor layer 32 and the insulating layer 31; and a conductive layer 34 which serves as a gate electrode is formed over the insulating layer 33.
In FIGS. 24A to 24D, when the insulating layer 33 which serves as a gate insulating layer is formed over the selectively etched semiconductor layer 32, the film thickness of the insulating layer 33 becomes uneven at the end portion 25 of the semiconductor layer 32, resulting in poor coverage. In a section where the film thickness of the insulating layer 33 is thin, the electric field strength of the gate voltage increases, and this adversely affects the withstand voltage and reliability of the thin film transistor.
Further, the stress of the substrate and of each thin film is concentrated at the end portion 25 of the semiconductor layer 32, and this causes variation in element characteristics.
In order to ameliorate the reduction in coverage by the gate insulating layer which is caused by a step at the end portion of the semiconductor layer 32, a method in which an end portion of an active layer is tapered is used (for example, see Reference 2: Japanese Published Patent Application No. 2005-167207).
Meanwhile, in thin film transistors included in circuits which require high-speed operation, it is preferable that a channel length is short and the gate insulating layer is thin. Therefore, the gate insulating layer has a small thickness of several tens of nanometers.